It is well recognized that for certain applications it is advantageous that ethylene polymers have a broad molecular weight distribution (MWD). For example, in blow molding of articles such as bottles, broad MWD polymers are desirable because they exhibit better impact strength and have superior environmental stress-crack resistance (ESCR) compared to polymers with narrow molecular weight distribution. Likewise, HDPE films with broad MWD are desirable because they exhibit good tear and impact properties.
Various expedients have been proposed to prepare ethylene polymers with broad molecular weight distribution modifications to the catalyst system such as described in many patents, for example U.S. Pat. Nos. 4,657,998 and 4,569,975.
Lowery, Jr. et al., U.S. Pat. No. 4,263,422, disclose a process for polymerizing alpha-olefins in the presence of independently supported transition metal catalysts, one of which is a catalytically active inorganic halide (e.g., MgCl.sub.2 or TiCl.sub.3) supported catalyst containing titanium or vanadium and another of which is an organic oxide-supported chromium-containing catalyst.
In accordance with this invention, novel catalysts quite different from those previously proposed are provided for preparing ethylene polymers and copolymer, particularly high density polyethylene (HDPE) with a multimodal and broad molecular weight distribution. The unique character of these catalysts is reflected in the general belief that the essential components, a magnesium oxide supported Ziegler catalyst and chromium catalyst component should not be combined because poisoning of the chromium catalyst would result.